Download Reproduction in Angiosperms

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Reproduction in Angiosperms
Reproduction is the process through which species can propagate by increase in their
number. This propagation or continuity of species is essential both in time and space.
Reproduction is the means of perpetuation of species. In plants new individuals may be
produced from single parent or from two parents depending upon the type of
reproduction.
Modes of Reproduction:
The reproduction in angiosperms is mainly of two types:
(1) Asexual Reproduction and (2) Sexual Reproduction.
1. Asexual Reproduction: It is the process of reproduction in which the
new individuals are formed without the fusion of gametes.
2. Sexual Reproduction: It is the process in which new individuals are
formed by fusion of gametes.
There is one more common method of reproduction observed in
Angiosperms, i.e. vegetative reproduction or vegetative propagation.
In this process, regeneration of new plants takes place from portions of
vegetative organs of the plants e.g. root, stem, leaf. As the fusion of
gametes is not involved, vegetative propagation is also included under
asexual type of reproduction.
Characteristic features of Asexual reproduction:
(a) It is uniparental, as only single parent is involved in this type of reproduction.
(b) The unit of propagule is a specialized or unspecialized part of the parent.
(c) The propagule is formed from body (i.e. somatic) cell of the parent hence
described as somatogenic reproduction.
(d) There is no fusion of gametes.
(e) The reduction division (i.e. meiosis) is absent. Therefore the cell division is of
mitosis type.
(f) It is quick method of reproduction.
(g) A very large number of individuals can be produced by a parent.
Characteristic features of Sexual reproduction:
1. In this method the offsprings are form indirectly through the process of
formation and fusion of gametes. It results into formation of diploid oospore
which is fusion product.
2. This type of reproduction is generally biparental. But it can also be
uniparental.
3. The sexual reproduction occurs through special germinal cell, hence the name,
germinal reproduction.
4. These cells develop special structures called sex organs which in turn give
rise to gametes.
5. The reduction division (i.e. meiosis) occurs at some stage of life cycle.
6. The gametes which are haploid fuse to form diploid oospore.
7. The oospore in turn produces the offspring.
8. The number of offsprings are produced by sexual reproduction is smaller than
the number of young ones produced through asexual reproduction.
9. It is relatively a slow process.
10. The offsprings produced through sexual reproduction show variation hence
they are not just replicas.
Vegetative Reproduction:
The vegetative reproduction in angiosperms can be divided in two categories:
(A) Natural Methods and (B) Artificial Methods.
(A) Natural Methods:
The vegetative reproduction occurs naturally in many flowering plants. The
natural methods of plants multiplication in which somatic part of the plant body
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separates and gives rise to the whole plant body. Thus propagule can be any part
of the plant body such as root, stem, leaf, bud etc. which develops into new
independent plant when given suitable conditions. The detached propagule may
be root, stem, leaf or floral bud.
1.
Tuberous roots (e.g. Sweet potato): This is the example of
adventitious root which is modified for the purpose of storage root.
It swells due to storage of starchy food and attains a cylindrical
shape, hence the name tuberous root.
The fleshy root of sweet potato develops the bud which are known
as adventitious buds. Generally, the buds do not develop on roots
hence these buds are called adventitious buds. These buds gives rise
to new plant.
2.
Stem Tuber (e.g. Potato): This is the underground stem structure
participating in the function of vegetative propagation. In potato
aerial branches grow downwards and come to store food. They
become swollen due to stored food and become cylindrical in shape,
hence the name tuber. The food –laden swelling has got a corky
cover. The depressions on the tubers are known as “eyes” which are
the nodes with axillary buds. The ‘eye’ with a small piece of tuber
serves as the ‘propagule’ or ‘seed’ from which another potato plant
can grow, when placed in soil in suitable environment.
3.
Runner (e.g. Cynnodon – Harali): This is a modification of subaerial or creeping stem for vegetative propagation. Cynodon is a
small grass member thriving well in damp soil. It develop special
narrow green horizontal branches called ‘runner’s. These branches
develop at the base of crown of leaves. It travels some distance on
the ground, touch the soil and gives rise to adventitious roots at
intervals and gives rise to crown of leaves upwards. Thus, a new
plant is formed. Breaking of runners helps in vegetative
propagation. The other examples are Centella (Bramhi)
4.
Leaf (e.g. Bryophyllum – Panphuti): The leaves of this plants
show crenate margin. The buds lie in the notches of margin of the
leaf. When these leaves fall on to the soil, the buds in the margin
develop into new plants.
(B) Artificial Methods:
These are several artificial methods of vegetative propagation. These are manmade special techniques in which somatic body of a plant is made to develop
into new independent plant. These methods are used for propagation of
desired verities according to one’s requirements. The best seasons to carry out
these techniques are rainy and spring seasons. Accordingly there are various
horticulture methods for vegetative propagation. They are
(a) Cutting: Many plants like, sugarcane, Croton, Hibiscus rosa-sinensis, rose
etc. can be easily grown by means of stem cutting. The cutting can be defined
as, ‘the cut pieces of root, stem and leaves’. These are planted in nurseries in
natural polarized fashion. For successful cutting it is necessary to induce
rooting or development of adventitious roots.
Leaf cuttings (e.g. Sansevieria – Snake plant): This plants can be
propagated by using leaf cutting. For this purpose the leaves are cur in
transverse manner into two or tree parts. These parts are planted in soil in
vertical manner. For quick rooting the rooting hormone can be applied to the
end which is in the soil. In this technique the formation of adventitious buds is
of equal importance.
Root cutting: The root cuttings from lemon, tamarind etc. sprout
producing roots and shoots, if they are put in the soil. The other examples are
Blackberry, Raspberry etc. Here ability to form adventitious roots and
adventitious buds are the prerequisites.
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(b) Budding: In vegetative propagation by budding methods, only one bud
with a small portion of bark, with or without wood is utilized. The budding
can be defined as grafting one bud ‘scion’ on the selected root ‘stock’. The
use of budding is generally restricted to young plants and smaller branches of
large trees. In this process two pieces of living plant tissue are connected
together in such a manner, that they will unite. Subsequently they grow and
develop as one plant. The two pieces are in the form of:
1. Root Stock: The root stock is the trunk or root material to which, buds
or scions are grafted or budded. The ‘scion’ is a small shoot that is
inserted by grafting into a root stock.
2. The second piece is called single budded scion. The root stock used for
budding should have the desired characters like vigour, growth habit,
resistance to soil borne pests etc. The root stock may be a rooted
cutting or seedling which is properly established in soil.
The advantages of Budding are:
1. It results into stronger union between scion and stock.
2. As each bud can give rise to a new plant, this potential can be
fully utilized in budding.
3. The technique is simple and can be performed easily by the
amateur.
4. The budding is quick and efficient method. Thus, many buds
can be grafted within short period with higher percentage of
success.
The important methods of budding are: (1) Shield or ‘T’ budding,
Inverted ‘T’ budding (2) Patch budding.
Shield budding is used for propagation of many fruit trees and
species of many ornamental plants. e.g. Citrus, Pear, Peach and
Rose. The patch budding is used for species with thick bark e.g.
Rubber tree, Mango, Walnut etc. The proper season is early and
late summer.
Procedure: Scion is a bud with a small piece of bark and
cambium. The stock is given ‘T’ shaped cut. The bark is lifted to
expose cambium. The bud is then inserted and the bark is allowed
to come back to its normal position. The bud is then exposed. The
joint is then treated with grafting wax and wrapped with bandage.
The bud sprouts after 3-5 weeks. By this time the organic
connection as been developed. Leaves and buds of stock are
removed. Finally stock is cut above the bud graft.
(c) Grafting: It is practiced in plants which do not root easily. This is also
applicable for the plants processing a weak root system. Grafting can be
defined as, “The art of joining parts of two plants of the same species
(intraspecific) or between allied species (i.e. interspecific) in such a manner
that brings about fusion or organic union of tissues.” The main part is stock.
The scion is the portion which is to be grafted. The scion is selected from
plant having desirable or superior characters. The grafting is commonly
practiced in tree members like mango, apple, pear, Citrus, guava and rubber
plant.
Types of grafting: There are different types of grafting in practise. They are as follows:
(a) bud grafting (b) approach grafting (c) tongue grafting (d) wedge grafting (e) crown
grafting. These methods are based upon methods of uniting two parts.
Thus, in a typical grafting experiment, the part of the graft which gives rise to upper part
is scion and lower part is called stock. The graft or scion should have one to several buds.
The root system of the other plant is allowed to remain intact and called stock (rootstock,
understock). The shoot of the stock is often cut 10-30 cms above the base of root. The
leaves and buds present over the stock are removed. In tongue (Slice or whip) grafting,
oblique sloping cut or notch is given to both stock and scion. The two perfectly fit upon
one another. They are tied together. The stock and scion should be of same diameter. In
grafting, scion is fixed over the stock in such a manner that the cambia of the two come
in contact. The union is covered with the grafting wax. It is the, tied with the help of
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bandage, rubber or nail and wrapped with polythene tape. The buds of the stock are not
allowed to sprout. They should be removed as soon as noticed.
After few weeks (about 4 to 6) the cut surfaces unite and graft is successful. The strings
or bandage and tape is then removed to avoid girdling (i.e. destruction of an area of bark
around the periphery of stem). Now root system and shoot system of two different plants
connect in such a way that they unite and later develop as a composite plant.
Significance of Vegetative Reproduction:
Merits:
1. The vegetative reproduction is useful for propagating plants with genetic sterility.
e.g. Banana, Pine apple, Orange, Rose etc.
2. It brings about cultivation and perpetuation of seedless varieties of grapes.
3. It maintains useful characters in heterozygous plants.
4. It is convenient and easy method for propagation e.g. Mango Citrus.
5. It induces disease resistance e.g. fruits in Citrus.
6. Cultivation under unfavourable conditions. e.g. Avocado, Apple, Peach, Almond
varieties.
7. Change in growth habit can be achieved by using specific root stock e.g. Applethe same cultivar (cultivated variety) dwarf, semidwarf and tall plants have been
obtained.
8. Early flowering and fruiting can be achieved with the help of grafted variety as
compared to the same varieties propagated by seeds e.g. Mango trees.
9. It helps in pollination and to overcome self compatibility in case of orchard
plants.
10. It helps in production of ‘clones’ of useful cultivar.
Demerits:
1. Vegetative reproduction cannot produce new variety.
2. It is expensive method as compared to seed multiplication.
3. It requires special trained staff for carrying out method like grafting, budding etc.
4. The plants raised through vegetative propagation have short life span as compared
to those raised by seed cultivation.
5. The plants raised through vegetative reproduction show less anchorage and there
is possibility of uprooting during storm as these plants develop adventitious roots
whereas the seed cultivated varieties have tap root systems.
Sexual Reproduction:
It is a method of reproduction in which new individuals or offsprings are formed through
the formation and fusion of haploid gametes. The fusion results in production of diploid
zygote that grows to produce a new individual.
In flowering plants, reproductive structures are flowers.
The typical flower essentially consists of four different whorls present on knob shaped
structure called receptacle. These whorls are known as floral whorls, the details of which
we have studied in XI standard.
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These whorls are, calyx, corolla, androecium and gynaecium. The four whorls are divided
into two groups: (i) Non-essential whorls (e.g. calyx and corolla) and (ii) essential or
reproductive whorls (e.g. androecium and gynaecium). These whorls are directly
involved in the function of reproduction.
Development of male gametophyte:
Structure of Male gametophyte:
The male reproductive whorl is called androecium. It consists of stamens. The
number of stamens varies in different plants from one to many. The typical stamen is
further differentiated into filament, connective and anthers. The filament is the stalk like
part of the stamen. It raises the anthers to a proper height. The anthers are sac like lobes
generally two in number. They are attached to the filament. The connective is the narrow
strip of tissue which joins the anthers. Thus, in transverse section each anther shows two
chambers. Thus, we get tetralocular structure known as, microsporangium.
T. S. of anther shows presence of anther sac wall which encloses within it the
sporogenous tissue, in which diploid microscope or pollen mother cells are differentiated.
The pollen grains or microspores are formed from microspore mother cell after
undergoing meiotic division. At maturity anther dehisces to liberate pollen grains.
Thus, pollen grains are unicellular, uninucleate haploid in structure. It is the first cell of
male gametophyte. The pollen grains is covered by two coats, the outer is called exine
and inner is called intine. The exine is thick with smooth or spiny outer surface and the
intine is thin and smooth. The exine shows presence of one or more germ pores which are
meant for the exit of pollen tube.
The microspore enlarges and undergoes a mitotic division to form a large vegetative cell
and small generative cell. The structure representing the above male gametophyte is
known as, pollen grain. The pollen grain is generally shed at this two celled stage.
The pollen grains are then transferred to the stigma of proper flower. They are held by
sticky secretion. The pollen grains germinates on the surface of stigma and finds a
suitable surface. The pollen grains germinates to form pollen tube. The growth of the
pollen tube is stimulated by surgery secretions of the stigma. The pollen tube is formed as
cytoplasmic outgrowth called germ tube and continues to grow into pollen tube. The
vegetative nucleus moves to the tip of the tube. It is followed by the generative nucleus.
The pollen tube transverses through stigma and style and reaches the ovary. The
generative nucleus of pollen tube divides mitotically to produce two male nuclei or male
gametes. This completes the development of male gametophyte.
Structure of Female Gametophyte:
The gynaceium is the inner most whorl of the flower and consists of carpels. The carpel
is comparable with megasporophyll which has folded along the midrib and the two
margins unite. The midrib portion of carpel is known as, dorsal suture and fused margins
correspond to ventral suture. The placenta bears ovules.
The typical arpel consists of ovary, style and stigma. The ovary is basal, swollen part of
the carpel and contains more or one ovules. The style is present at the top of the ovary. It
bears stigma at its tip. The style raises the stigma to proper heights which facilitates the
pollination. The ovary after pollination and fertilization develops into fruit and ovules are
transformed into seeds.
Structure of Ovule:
The typical ovule is the megasporangium with its protective coats, the integuments, and
is known as ovule. The typical ovule is attached to placenta on the inner wall of ovary.
The ovule is attached to the ovary wall by a stalk called funiculus. In anatropous ovule
the micropylecomes to lie close to the funiculus due to the unilateral growth of the ovule.
It is also known as inverted ovule. This is the most common type of ovule in angiosperms
and hence it is studied in detail.
Structure of Anatropous Ovule: The typical anatropous ovule in vertical section shows
following parts.
(i)
Nucellus: It is the main central part of the ovule made up of diploid
parenchymatous tissue.
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(ii)
Integuments: The nucellus is surrounded by one or two wall layers
called integuments. The ovule with one integument is termed as
unitegmic while with two integuments is termed bitegmic.
(iii) Micropyle: The integuments do not cover the ovule completely, but
leave a small space, called micropyle. In the anatropous ovule,
micropyle lies near the funicle. It helps in the entry of pollen tube.
(iv)
Funicle: It is the stalk of the ovule by which the ovule is attached with
the placenta.
(v)
Hilium: The point of attachment of the body of the ovule to its stalk
i.e. funicle is called hilum.
(vi)
Raphe: In anatropous ovule, the funicle continues beyond the hilum
alongside the body of the ovule forming a sort of ridge, called raphe. It
carries the food to the nucellus.
(vii) Chalaza: The distal end of the funicle which is the junction of the
integuments and nucellus is called the chalaza.
(viii) Embryo sac: It is developed in the nucellus. It is also called as female
gametophyte, where the embryo is formed after fertilization.
Development of female gametophyte:
The diploid megaspore mother cell becomes differentiated in the nucellus. It is slightly
larger than the other cells of nucellus. It undergoes reduction division and produces a row
of 4 haploid megaspores. Out of 4, upper three degenerate and only one megaspore
becomes functional. It soon enlarges in size and forms embryo sac or female
gametophyte. The nucleus in the megaspores undergoes three mitotic divisions to form 8
nuclei. These get arranged in two groups of 4 nuclei each, towards micropylar end and
chalazal end. The 3 nuclei at the micropylar end form egg apparatus containing as egg
cell and two synergid cells. Similarly three nuclei at calazal end form 3 antipodal cells.
One nucleus from each pole (i.e. micropylar end and calazal end) get arranged in the
centre to form polar nuclei. They fused to form secondary nucleus. In the straight ovule
(i.e. orthotropous ovule) the hilum, chalaza and micropyle lie in a straight line.
Pollination Types and Agencies:
Pollination: Definition: It is the process of transfer of pollen grains from anthers of a
flower to the stigma of the same or a different flower and is called pollination. The
pollen grains may be transferred to the stigma of a flower present on the same plant or
other plant of the same species.
The pollen grains are nor motile. As such they cannot move themselves to the receptive
stigma of the same or different flower. The pollination helps the transfer of pollen grains
to the stigma. Thus, pollination forms as integral part of sexual reproduction in
Angiosperms.
Types of pollination: The pollination is of two types: (i) Self pollination and (ii) Cross
pollination.
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(i) Self pollination: In this type of pollination the pollen grains from the anther are
transferred to the stigma of same flower or another flower present on the same plant e.g.
sugarcane, Commelina.
Autogamy is the process in which pollen grains are transferred to the stigma of same
flower. Geitonogamy is the process in which pollen grains are transferred to stigma of
different flower on the same plant.
(ii) Cross pollination: In this type of pollination the pollen grains are transferred from
anthers to the stigma of the other flower belonging to the same species. Xenogamy is the
process in which pollen grains are transferred to the flower on separate plant of the same
species. Geitonogamy is the process of cross pollination taking place in between two
different flowers on the same plant.
Pollinating agencies: The pollen grains are non-motile, therefore, cross pollination
involves some agencies to carry the pollen grains from the anther to the stigma. Such
agencies which carry the pollen grains are called pollinating agents.
Types of pollinating agents: The pollinating agents are of two types: (A) Biotic and (B)
Abiotic.
(A) Biotic Pollinating agents: These are biotic components in the surrounding
atmosphere and are therefore in the form of different living systems, like animals,
insects etc. As per the nature of pollinating agents, the different terms are used
e.g.,
1. Zoology
→ flowers are pollinated by animals
2. Entomophily → flowers are pollinated by insects
3. Ornithophily → flowers are pollinated by birds
4. Chiropteriphily → flowers are pollinated by bats
5. Malacophily
→ flowers are pollinated by snails.
(a) Ornithophily: When the flower is pollinated by bird, the
phenomenon is known as ornithophily. In some part of the
tropics, birds are important pollinators as compared to insects.
The common birds participating in the pollination are Hummingbirds, sun-birds, and honey eaters. The ornithophilous flowers
are: (1) Generally tubular (Nicotiana gluca), cup shaped
[Callistemon (Bottle brush)]. (2) They are bright coloured. (3)
They produce large quantities of pollen and nectar. (4) Their
stamens and carpels (i.e. pistils) produce beyond the perianth
lobes. (5) The red, orange, yellow colours attract-birds from long
distances e.g. Red silk cotton tree, Coral tree, bottle brush etc. In
Sterlitzia reginae pollination is brought about by sun-birds. The
other common examples are, Bignonia, Sanchezia etc.
(b) Entomophily: When the flower is pollinated by insects, the
phenomenon is known as entomophily. Insects are chief
pollinators and show various types of intimate relationship with
flowers they visit. The insects which help in pollination are bees,
flies, wasps, moths, beetles. The bees flies and beetles are
diurnal as they are active in daytime whereas moths are
nocturnal i.e. they are active during night hours. The bees are
the main flower visitors and bring about the pollination to the
tune of 80 per cent of all pollination done by insects.
The insect pollinated flowers show the following
characters: (1) The flowers are brightly coloured. (2) They
possess fragrance and/or produce nectar. (3) The bees are fond of
yellow, violet and purple coloured flowers. (4) The bees while
collecting their food in the form of nectar and pollen, visit the
flowers and bring about pollination. (5) Some flowers emit
unpleasant smell e.g. Arum, Rafflesia etc. Some flies get
attracted towards this smell and further bring about the
pollination. (6) The orchid, Ophyrus speculum is pollinated by
hairy wasp. This orchid flower exactly resembles female
counterparts. Also these flowers possess an appearance and
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odour similar to those possessed by the female wasp. Thus,
males mistakes the Ophyrus flowers for their counterpart, land
upon them to perform the act of pseudo-copulation. They repeat
his pollen grains are spiny while the stigma is sticky. (8) Moths
generally visit flowers at night. e.g. Salvia Helianthus etc. (9)
The typical lever mechanism is seen in Salvia.
(B) Abiotic Pollinating Agents: These are the non living agencies and hence passive
in nature. The abiotic pollinating agents are of two types: (i) Wind and (ii) Water.
(c) Anemophily: When the flower is pollinated by wind the
phenomenon is known as ‘anemophily’. The common
examples are Grasses, Maize, Jowar etc. In anemophily the
air currents pick up the pollen grains from the air currents.
The characters of anemophilous flowers are:
(1) They produce large number of pollen grains as there is
possibility of wastage of quite a large number of pollen
grains due to passive nature of wind.
(2) The flowers are small and incospicous.
(3) The perianth lobes are reduced or absent.
(4) The pollen grains are small, light, smooth and dry.
(5) The male flowers generally occupy terminal position by
means of which wind can easily blow away the
discharged pollen. The fixation of anthers to the
filament is versatile. Thus, anthers can freely oscillate
in all the directions at the tip of filament, according to
wind current.
(6) In female flowers which are in axile position in Maize
plant are present at lower position which is convenient
for catching the pollen grains.
(7) Also the stigma is feathery and long. The flowers are
unisexual, pollen grains small in size, smooth and dry.
The other examples are: coconut palm, Datepalm,
Cannabis (Bhang)
(d) Hydrophily: When the flower is pollinated by water, the
phenomenon is known as hydrophily. The examples of water
pollinated flowers are, Vallisneria, Lemna, Ceratophyllum,
Zostera etc. The water pollination occurs in the plants which
are totally submerged marine plants such as Zostera marina.
This type of water pollination phenomenon is called
hypohydrophily or hyphydrophily. In Caretophyllum, the
male flower contains 30-40 stamens. The anthers at maturity
get separated at the base. Then they float to the surface of
water. They dehisce and liberate the pollen grains. The
released pollen grains sink and germinate. The pollen grains
then pollinate the female flowers. The important
characteristics of hydrophilous flowers are:
(1) The pollen grains in water pollinated plants get
themselves adapted to help water pollination. Thus, in
Zostera marina pollen grains are exceptionally long
and needle like and resemble pollen tube.
(2) The specific gravity of pollen grains is nearly same to
that of marine water. Therefore, the pollen grains can
freely float in water at any depth.
(3) The stigma of such plant is very long. When pollen
grain comes in contact with stigma it coils around the
stigma.
Vallisneria plant is submerged and dioecious plant. The
flowers are borne under water. The male flowers get
detached at maturity from parent plant and float on
surface of water. The female flowers develop under
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water.
(4) At the time of pollination they are brought to the surface
of water with the help of long slender stalk.
(5) The female flower ultimately gets arranged in
horizontal position. It creates a cup-shaped depression
around it due to its weight. The male flower floating on
the water surface gets into the depression formed by
female flower. The anthers come in contact with
exposed stigma to transfer pollen masses to it.
(6) The pollen grains are usually without an axine as
observed in Vallisneria, Certaphyllum and Zostera.
The stalk of the female flower after pollination
undergoes spiral twisting and ultimately brings the
pollinated female flowers under the surface of water.
When the flower is pollinated by water above its
surface, the phenomenon of hydrophily is further
described as epihydrophily or ephydrophily.
Necessity and Significance of Pollination:
(1) The act of pollination further paves its way to fertilization.
This gurantees the production of seeds and fruits.
(2) The products of pollination and fertilization i.e. seeds and
fruits, in turn serve as a source of food for animals
including human beings.
(3) The cross pollination creates a chance of combination of
characters of two plants. It further helps in the process of
evolution.
(4) In the process of hybrid seed production the role of
pollination is very significant.
Fertilization:
Fusion of male gamete with the female gamete is called fertilization. In pollination the
pollen grains reach the stigma and start their germination. The pollen is shed from the
anther at 2-celled condition (i.e. small generative cell and large vegetative cell or tube
cell). Then the exine ruptures and intine grows through the germ pore into a tube celled
pollen tube. The pollen tube grows downward through the stigma into the style,
ultimately reaches the ovule through micropyle. The generative nucleus divides into two
male gametes. The tube nucleus later on disorganizes.
Usually, the pollen tube enters the ovule through the micropyle. The process is known as,
porogamy. The pollen tube may also enter through chalaza (chalazogamy) or through
integuments (misogamy). Each male gamete consists of a nucleus surrounded by a sheath
of cytoplasm.
The male gametes are carried at the tip of pollen tube. The pollen tube reaches the ovule,
penetrates the embryo sac. The tip of the pollen tube bursts to liberate the two gametes
near the egg cell.
One of the male nucleus fuses with egg and forms the diploid oospore. Thus, fusion of
male gamete with the female gamete or egg is known as, fertilization.
The second male gamete fuses with the secondary nucleus and forms the primary
endosperm nucleus. It is noteworthy that two polar nuclei or secondary nucleus is diploid
(n + n). When male gamete fuses with secondary nucleus the product – primary
endosperm nucleus becomes triploid (i.e. 2n + n = 3n). Therefore, this fusion is also
known as, triple fusion forming primary endosperm nucleus.
Significance:
(1) Double fertilization is unique feature of Angiosperms. During this process
both the gametes released by pollen tube are utilized in fertilization.
(2) The two male gametes fertilize: (a) haploid egg, forming diploid zygote which
ultimately develops into a diploid embryo. (b) The secondary nucleus (2N),
forming a triploid primary endosperm nucleus. It ultimately gives rise to an
endosperm, which is a specialized nutritive tissue.
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(3) Due to double fertilization, the embryo is supplemented with nutritive tissue,
the endosperm. The endosperm thus ensures the growth and development of
embryo. The endosperm in Angiosperms is post-fertilization product. (In
gymnosperm endosperm is always haploid and formed before fertilization).
Endosperm and Embryo Development:
Development of endosperm:
The formation of endosperm is initiated by mitotic divisions of primary
endosperm nucleus (3n). Endosperm is a special nutritive tissue. The fusion of
male gamete (n) and secondary nucleus (2n) results in formation primary
endospermic nucleus (PEN) (3n). PEN undergoes many mitotic divisions to
form triploid endosperm. It fills up the space of nucellus. The formation of
endosperm occurs usually prior to the zygotic or oospore division.
The endosperm is of three types: viz. (i) nuclear (ii) cellular and (iii) helobial. In
the nuclear type, the nucleus undergoes repeated divisions. The nuclei do formed
get arranged towards the periphery, leaving a large central vacuole. Then
cytokinesis follows from periphery to the centre. Thus at maturity it shows
cellular appearance. This, is common type of endosperm as seen in Maize,
Sunflower, Wheat.
2. Cellular Type: Here every nuclear division is followed by cytokinesis,
thus, giving cellular appearance from the beginning. e.g. Datura.
3. Helobial Type: Here the first mitotic division is followed by cytokinesis.
This results in the formation of two unequal cells. The divisions taking
place in both the cells are free nuclear. e.g. Asphodelus tenuifolius.
The seeds in which endosperm is present are known as endospermic or
albuminous seeds. In many seeds, however all the food stored up in the
endosperm at its early stages is used by developing embryo. The
endosperm is thus absent in seeds. Therefore, they are called nonendospermic seeds or exabluminous seed. The examples of endospermic
seeds are Jowar, Castor etc. and non – endospermic seeds are Pea, Bean,
Groundnut etc.
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XII
Development of Embryo:
Fusion of male gamete with the egg cell results in the formation of zygote.
The first division of the zygote produces a hypobasal cell (basal) towards the micropyle,
and epibasal cell (terminal) towards chalaza. The basal cell divides repeatedly and
ultimately produces a row of 4-8 cells and forms the suspensor. The terminal cell divides
first vertically and then transversly forming 4-celled proembryo. The suspensor pushes
the proembryo into the endosperm to enable developing embryo to receive nutrition. The
suspensor absorbs food and feeds the developing embryo. The main aim of suspensor is
to push the young embryo deeper into the embryo sac.
The basal cell of suspensor enlarges and acts as an absorbing organ. It is called
haustorial cell while its terminal cell called hypophysis cell, divides and redivides and
gives rise to the apex of radicle.
The proembryo by further divisions gives rise to whole of the embryo with all of its
differentiated parts i.e. the radicle, plumule (stem apex) and two cotyledons (in dicot
seed) or only one cotyledon (in monocot seed).
The relative positions of micropyle, suspensor, embryo with cotyledons, endosperm can
be understood by referring to (H.I.). As the endosperm and embryo mature, the
integuments of ovule become hard. Thus, they provide protective covering called seed
coat, i.e. testa. Finally there is formation of seed.
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